Image recording systems write digital data onto photosensitive media by applying light exposure energy. Such energy may originate from a number of different sources and may be modulated in a number of different ways. Image recording systems can be used for digital printing, whereby digital image data is used to print an image onto photographic paper or film.
One of the early methods used for digital printing was cathode ray tube (CRT) based systems. In a CRT based printer, the digital data is used to modulate a CRT, which provides exposure energy by scanning an electron beam of variable intensity on a phosphorescent screen. This technology has several limitations related to the phosphor and the electron beam. The resolution of this technology is inadequate when printing a large format image, such as 8 inch by 10 inch photographic prints. CRT printers also tend to be expensive, which is a severe shortcoming in cost sensitive markets such as photoprocessing and film recording. An additional limitation is that CRT printers do not provide sufficient red exposure to the media when operating at frame rates above 10,000 prints per hour.
Another commonly used approach to digital printing is the laser based engine shown in U.S. Pat. No. 4,728,965. Digital data is used to modulate the duration of laser on-time or intensity as the beam is scanned by a rotating polygon onto the imaging plane. Such raster scan systems use red, green, and blue lasers. Unfortunately, as with CRT printers, laser based systems tend to be expensive, since the cost of blue and green lasers remains quite high. Additionally, there is limited availability of compact lasers with sufficiently low noise levels and stable output for accurate, artifact-free imaging.
Due to reciprocity failure, photographic paper and film media are not suitable for color imaging when using a laser light source. High intensity reciprocity failure is a response phenomenon by which both photographic paper and film are less sensitive when exposed to high intensity light for a short time period. For example, raster scan laser printers expose each pixel within an image frame for a fraction of a microsecond, whereas optical printing systems expose the full image frame on paper or film for a longer duration, typically on the order of seconds. Thus, special paper and film are required for laser printers.
In an effort to reduce cost and complexity of printing systems, as well as avoid reciprocity failure, alternative technologies have been considered for use in digital printing. Among suitable candidate technologies under development are two-dimensional spatial light modulators.
Two-dimensional spatial light modulators, such as the digital micromirror device (DMD) from Texas Instruments, Dallas, Tex., or a liquid crystal device (LCD) can be used to modulate an incoming optical beam for imaging. A spatial light modulator can be considered essentially as a two-dimensional array of light-valve elements, each element corresponding to an image pixel. Each array element is separately addressable and digitally controlled to effect image modulation by manipulating the polarization state of light, thereby allowing or blocking transmission of incident light from a light source. Polarization considerations are, therefore, significant to the overall design of support optics for a spatial light modulator.
There are two basic types of spatial light modulators currently in use. The first type developed was the transmission spatial light modulator which, as its name implies, operates by selective transmission of an incident optical beam through individual array elements. The second type, a later development, is a reflective spatial light modulator. As its name implies, the reflective spatial light modulator, operates by selective reflection of an incident optical beam through individual array elements. A suitable example of an LCD reflective spatial light modulator relevant to this application utilizes an integrated Complimentary Metal Oxide Semiconductor (CMOS) backplane, allowing a small footprint and providing improved uniformity characteristics.
Spatial light modulators provide significant advantages in cost, as well as avoiding reciprocity failure. Spatial light modulators have been proposed for a variety of different printing systems, from line printing systems such as the printer disclosed in U.S. Pat. No. 5,521,748, to area printing systems such as the system disclosed in U.S. Pat. No. 5,652,661.
A single spatial light modulator such as a Texas Instruments digital micromirror device (DMD) disclosed in U.S. Pat. No. 5,061,049, can be used for digital printing applications. One approach to printing using the Texas Instruments DMD, shown in U.S. Pat. No. 5,461,411, offers advantages such as longer exposure times using light emitting diodes (LED) as a source. Thus, reciprocity problems associated with photosensitive media exposed for short periods are eliminated. However, DMD technology is expensive and DMD devices are not widely available. Furthermore, DMDs are not easily scaleable to higher resolutions, and the currently available resolution is not sufficient for all digital printing needs.
Several photographic printers using commonly available LCD technology are described in U.S. Pat. Nos. 5,652,661, 5,701,185, and 5,745,156. Most of these designs utilize a transmissive LCD modulator such as those depicted in U.S. Pat. Nos. 5,652,661 and 5,701,185. While such methods offer several advantages in ease of optical design for printing, there are several drawbacks to the use of conventional transmissive LCD technology. Transmissive LCD modulators generally have reduced aperture ratios. Thin Film Transistors (TFT) on glass technology does not yield the pixel to pixel uniformity required in many printing and film recording applications. Furthermore, in order to provide large numbers of pixels, many high resolution transmissive LCDs have a large physical footprint. With footprints of several inches, such devices can be difficult to adapt when combined with a lens designed for printing or film recording applications. As a result, most LCD printers using transmissive technology are constrained to either low resolution or small print sizes.
To print high resolution 8 inch by 10 inch images with at least 300 pixels per inch requires 2400 by 3000 pixels. Similarly, to print high resolution images onto film requires at least 2000 by 1500 pixels, and can require as much as 4000 by 3000 pixels. Transmissive LCD modulators with sufficient resolution for such printing applications are not readily available. Furthermore, the grayscale depth needed for each pixel in order to uniformly render a continuous tone print over the frame size is not available with this technology.
The use of a reflective LCD serves to significantly reduce the cost of the printing system. Furthermore, the use of an area reflective LCD modulator sets the exposure times at sufficient length to minimize or eliminate reciprocity failure. The progress in the reflective LCD device field made in response to needs of the projection display industry have provided opportunities in printing applications. Thus, a reflective LCD modulator designed for projection display can be incorporated into a printer design with little modification to the LCD itself. Also, designing an exposure system and data path for a printer using an existing projection display device allows incorporation of an inexpensive commodity item into the print engine.
For printing applications, the most suitable reflective LCD devices incorporate a small footprint with an integrated CMOS backplane. The compact size along with the uniformity of drive offered by such a device translates into better image quality than is available using other LCD technologies. There has been progress in the projection display industry towards incorporating a single reflective LCD, primarily because of the lower cost and weight of single device systems. See, for example, U.S. Pat. No. 5,743,612. Of the LCD technologies, the reflective LCD with a silicon backplane can best achieve the high speed required for color sequential operation. While this increased speed may not be as essential to printing as it is for projection display, higher speeds can be utilized to incorporate additional grayscale and uniformity correction into printing systems.
The recent advent of high resolution reflective LCDs with high contrast in excess of 100:1, presents possibilities for printing that were previously unavailable. See, for example, U.S. Pat. Nos. 5,325,137 and 5,805,274. Specifically, a printer may be based on a reflective LCD modulator illuminated by a lamp, by lasers, or by an array of red, green, and blue light emitting diodes. The reflective LCD modulator may be sub-apertured and dithered in two or three directions to increase the resolution.
Reflective LCD modulators have been widely accepted in the display market. Most of the activity in reflective LCD modulators has been related to projection display, such as is disclosed in U.S. Pat. No. 5,325,137. Several projector designs use three reflective LCD modulators, one for each of the primary colors, such as the design shown in U.S. Pat. No. 5,743,610.
It is instructive to note that imaging requirements for projector and display use (as is typified in U.S. Pat. Nos. 5,325,137; 5,808,800; and 5,743,610) differ significantly from imaging requirements for digital printing onto photographic paper or film. Projectors are optimized to provide maximum luminous flux to a screen, with secondary emphasis placed on characteristics important in printing, such as contrast and resolution. To achieve the goals of projection display, most optical designs use high intensity lamp light sources. Optical systems for projector and display applications are designed for the response of the human eye, which, when viewing a display, is relatively insensitive to image artifacts and aberrations and to image non-uniformity, since the displayed image is continually refreshed and is viewed from a distance. However, when viewing printed output from a high-resolution printing system, the human eye is not nearly as forgiving to artifacts and aberrations and to non-uniformity, since irregularities in optical response are more readily visible and objectionable on printed output. It is instructive to note that the gamma for human eye response when viewing projected images in a dark room is approximately 0.8. In contrast, the gamma when viewing printed output in normal lighting is approximately 1.6. As a result, small artifacts are more easily visible in printed images than in projected images, complicating the task of providing uniform exposure energy for printing applications. It is also instructive to note that projectors are typically designed for presentation of motion images. With motion images, due in part to varying image content and artifact motion, image variations are not easily perceptible to the human eye. This is in contrast to stationary images, in which artifacts tend to be stationary and, therefore, more visible.
Even more significant differences between the projection and printing imaging environments are differences in resolution requirements. Adapted for the human eye, projection and display systems are optimized for viewing at typical resolutions such as 72 dpi or less, for example. Apparatus used for printing onto photographic paper or film must achieve much higher resolution, particularly apparatus designed for micrographics applications, which can be required to provide 8,000 dpi for some systems. Resolution of a printed image can be enhanced by image displacement, by dithering, or by performing multiple exposures. The short time interval that exists between the display of different images makes increasing the resolution by using these techniques impractical. Thus, while LCD spatial light modulators can be used in a range of imaging applications for projection and display to high-resolution printing, the requirements for supporting optics can vary significantly.
A preferred approach for digital printing onto photographic paper and film uses a reflective LCD based spatial light modulator. Liquid crystal modulators can be a low cost solution for applications requiring spatial light modulators. Photographic printers using commonly available LCD technology are disclosed in U.S. Pat. Nos. 5,652,661; 5,701,185; and 5,745,156.
Although the present invention primarily addresses use of reflective LCD spatial light modulators, references to LCDs in the subsequent description can be generalized, for the most part, to other types of spatial light modulators, such as the previously noted Texas Instruments DMD device.
Primarily because of their early development for and association with screen projection of digital images, spatial light modulators have largely been adapted to continuous tone (contone) color imaging applications. Unlike other digital printing and film recording devices, such as the CRT and laser based devices mentioned above that scan a beam in a two-dimensional pattern, spatial light modulators image one complete frame at a time. Using an LCD, the total exposure duration and overall exposure energy supplied for a frame can be varied as necessary in order to achieve the desired image density and to control media reciprocity characteristics. Advantageously, for printing onto photographic paper and film, the capability for timing and intensity control of each individual pixel allows an LCD printer to provide grayscale imaging.
Modulator printing systems can incorporate a variety of methods to achieve grayscale. Texas Instruments employs a time delayed integration system that works well with line arrays as shown in U.S. Pat. Nos. 5,721,622, and 5,461,410. While this method can provide adequate gray levels at a reasonable speed, however, line printing Time Delayed Integration (TDI) methods can result in registration problems and soft images. Alternate methods have been proposed particularly around transmissive LCDs such as the design presented in U.S. Pat. No. 5,754,305.
Dithering has been applied to transmissive LCD systems as one way to correct for a less than perfect fill factor. Incorporating dithering into a reflective LCD printing system would allow high resolution printing while maintaining a small footprint. Also, because the naturally high fill factor present in many reflective LCD technologies, dithering action can be omitted with no detriment to the continuity of the printed image.
Alternative forms of optical dithering are used to improve resolution in display systems incorporating LCD modulators. For example, a calcite crystal or other electro-optic birefringent material can be used to optically shift the path of an image beam, where the degree of shift is dependent on polarization characteristics of the image. This allows shifting one component of an image with respect to a second component of the image that has a different polarization. See U.S. Pat. Nos. 5,715,029 and 5,727,860. In addition to the use of birefringent material, U.S. Pat. No. 5,626,411 employs the principle of refraction with isotropic optical media having different indices of refraction used to displace one image component from a second image component. These methods of beam displacement are used in a dynamic imaging system and serve to increase resolution by interlacing raster lines in order to form two lines of sub-images. The two sub-images are imaged faster than is perceivable to the human eye, so that the individual images are integrated into a composite image as seen by the observer. While these methods are appropriate for projection imaging systems, they are not suitable for a static imaging system such as printing.
While the reflective LCD modulator has enabled low cost digital printing on photosensitive media, the demands of high resolution printing have not been fully addressed. For many applications, such as imaging for medical applications, resolution is critical. Often, the resolution provided by a single reflection LCD modulator is insufficient. It then becomes necessary to form an image by merging multiple images in order to create a single high-resolution image. It is preferable to form such a merged image without artifacts along border areas or in regions where image data overlaps. While juxtaposing or spatially interweaving image data alone may have been attempted in previous applications, such superposition of images with the use of reflective LCDs provides images of high quality without compromising the cost or productivity of the print engine. Further, by utilizing polarization based modulation, a print engine can utilize light already available in the optical system.
Juxtaposing or spatially interweaving image data has been attempted with some success in projection displays. U.S. Pat. No. 5,715,029, describes a method to improve resolution of a display by altering the beam path using a birefringent medium such as a calcite crystal or an electro-optic liquid crystal cell. For projection applications using a transmissive LCD, Philips Corporation deflects the beam path by using birefringent elements as is disclosed in U.S. Pat. No. 5,727,860. Another method, using isotropic optical elements to juxtapose or spatially interweave images in a projection display using a transmissive LCD, is described in U.S. Pat. No. 5,626,411.
Thus, it is desirable to have a low cost, high-resolution, high speed method for digital printing onto a photosensitive media that avoids reciprocity failure and preserves adequate grayscale.